1. Field of the Invention
The present invention relates to flow meters, and in particular to a variable area obstruction in a conduit for use with a pressure transducer, particularly in medical applications, for measuring bi-directional flow rate of fluids such as respiratory gas in the conduit.
2. Prior Art
Long term monitoring of respiratory air flow in critical care patients and in patients during anesthesia is very important for correctly assessing the patient's condition and for selecting the course of his or her future treatment.
The conditions under which this monitoring occurs are not always ideal. For example, in order to monitor this respiratory air flow, a flow meter is typically installed in a breathing conduit in close proximity to a patient. Thus, the flow meter is exposed to a flow of warm air at body temperature that is 100% saturated with water vapor and contains airborne droplets of water, saliva or mucus. These airborne droplets can collect in the flow meter and adversely affect its operation. In addition, since the ambient temperature is always lower than body temperature, water vapor normally condenses on inner surfaces of conduits installed in breathing circuits. The water droplets formed by this condensation glide along the inner walls of the conduit, and frequently collect in, and impede operation of, the flow meter.
Of course, because of the necessity of very accurate and reliable measurements under these circumstances, it is important that the flow meter which is selected meet all of the requirements for proper operation under critical care conditions. In particular, these conditions require use of a very light weight flow meter, with small dead space, with a wide measuring range, and with an accuracy that is not affected by the presence of fluid, including mucus, produced by the patient.
When used in applications where a wide range of breathing flow rates are to be measured, it is very desireable that the flow meter response, as reflected in its output signal, be substantially linear, so that the sensitivity of the meter and accuracy of its readings is the same at low fluid flow rates as at high fluid flow rates. Alternatively, when very low flow rates are being measured, then it is often desireable to produce a flow meter response which changes substantially in response to small changes in the flow rate, thereby enabling more accurate monitoring of the measured flows. In addition, sterility requirements in critical care applications, such as operating room, require that the flow meter be disposable. This dictates a very low production cost, while still necessitating a high sensitivity and accuracy for each individual flow meter.
There are a number of different types of flow meters that are well known among those in the technology and which are acceptable for short term use in applications such as diagnostic pulmonary measurements, due to their linear output signal with respect to the measured flow characteristics. One such device is generally referred to as the Fleisch pneumotachograph, which is the most widely used flow meter for medical applications. This device directs the flow of gas through a bundle of long, small diameter tubes which laminarize the flow. Under laminar flow conditions, the pressure differential is linearly proportional to flow rate. Accordingly, the output signal is generally linear in its characteristics. The sensitivity also remains substantially the same, throughout the range of measurement. However, this device cannot be used for the long term monitoring of respiratory air flow, since moisture or mucus collecting on the bundle of small diameter tubes produces a significant, adverse effect on the output signal of the device. Also, because of the linear response produced throughout the entire range of measurement, this device is not suited for application where a non-linear response is desireable at low flow rates, with a linear response produced at higher flow rates.
Another problem associated with the Fleisch pneumotachograph is its mechanical complexity, which in turn requires a high production cost. The high production cost makes the unit essentially non-disposable. Because it is not disposable, the requirement that the device be cleaned and sterilized after each patient use adds additional cost to the use of this device. The problems encountered in use of the Fleisch pneumotachograph are also experienced in the use of other popular flow meters, such as the ultrasonic, hot wire and turbine pneumotachographs.
Other types of flow meters may be of simple design, but are either too bulky for use in critical care applications, or they produce a non-linear output signal in a flow range where a linear signal is desired, or they have a limited measuring range. These types of flow meters include the fixed orifice, venturi tube and pitot tube meters. More specifically, the fixed orifice flow meter provides an output signal defining a curve which gets progressively steeper as the flow rate increases. This is true since the device operates under turbulent flow conditions, where the pressure differential is proportional to the square of flow rate. This characteristic is undesirable because of drastically reduced sensitivity at low flow rates. Thus, fixed orifice flow meters are generally used in applications having a limited range of flow rates which are likely to be encountered.
Variable obstruction flow meters have also been provided, which combine the simplicity and low cost of the fixed orifice flow meters with better low end sensitivity and linear characteristics that typically have been available only from pneumotachographs. Nevertheless, many existing variable area obstruction designs still suffer from certain drawbacks such as accumulation of moisture or liquids in areas immediately adjacent the variable obstruction. These flow meters have also been known to suffer from resonant vibration or flutter of the leaves which comprise the variable obstruction at low flow rates corresponding to resting breathing of the patient. Furthermore, reduced sensitivity is often experienced at low flow rates due to relatively large leakage area in the obstruction produced by the gaps between the flexible leaves which are apparent when the leaves are in their resting condition. The large leakage area is usually a result of the type of material selected for use as the obstruction, and the manufacturing process selected for producing the obstruction. The material utilized for the obstruction will also determine the obstructions' predisposition to errors caused by unwanted deflection of the leaves due to gravitational or inertial forces.
Based on the above, it would be an important improvement in the art to provide a flow meter having a wide measuring range and accuracy, and which is substantially not affected by the presence of moisture or mucus produced by a patient. It would be a further improvement in the art to provide one embodiment of such a flow meter in which the pressure differential across the obstruction is approximately linearly proportional to the flow rate of fluid in the conduit at very low flow rates as well as at higher flow rates. It would also be an improvement in the art to provide another embodiment of such a flow meter in which the pressure differential across the obstruction varies non-linearly with respect to changes in fluid flow rate in the conduit at very low flow rates, but is approximately linearly proportional to such flow rate changes at higher flow rates. It would be a still further improvement in the art to provide such a flow meter wherein the size of the flow passage in the variable area obstruction is minimized at zero flow condition. It would be a still further important improvement in the art to provide such a flow meter which has a light, simple and inexpensive structure, permitting it to be disposable, while being manufactured in a manner that permits mass production and also maintains the high level of sensitivity and accuracy in each unit produced. A still further improvement in the art would be to provide such a flow meter where the active area of the variable obstruction is substantially the same as the full cross sectional area of the fluid conduit, and the elements of the variable obstruction are shaped in such a manner that they, at maximum deflection, leave a substantially unobstructed passage along the fluid conduit wall.